Externally or internally blocked polyisocyanates which are solid at room temperature, are valuable crosslinkers for thermally crosslinkable polyurethane (PU) powder coating compositions and adhesive compositions. For instance, DE-A 27 35 497 describes PU powder coatings having outstanding weathering stability and thermal stability. The crosslinkers described in DE-A 27 12 931 are composed of ε-caprolactam-blocked, isophorone diisocyanate-containing isocyanurate groups. Also known are polyisocyanates containing urethane, biuret or urea groups, whose isocyanate groups are likewise blocked.
The drawback of these externally blocked systems lies in the elimination of the blocking agent during the thermal crosslinking reaction. Since the blocking agent may be emitted into the environment, it is necessary to take particular measures to clean outgoing air discharges and to recover the blocking agent. This necessity arises out of ecological and workplace safety concerns. The crosslinkers, moreover, are of low reactivity, requiring curing temperatures above 170° C.
References DE-A 30 30 539 and DE-A 30 30 572 describe processes for preparing polyaddition compounds which contain uretdione groups, and whose terminal isocyanate groups are irreversibly blocked with monoalcohols or mono amines. Particular drawbacks are the chain-terminating constituents of the crosslinkers, which lead to low network densities in the PU powder coatings, and hence to moderate solvent resistances.
Hydroxyl-terminated polyaddition compounds containing uretdione groups are disclosed in EP 0 669 353. Because of their functionality of two, they exhibit improved resistance to solvents. Powder coating compositions based on these polyisocyanates containing uretdione groups, share the feature that, during the curing reaction, they do not emit any volatile compounds. However, the curing temperatures are at a high level, not less than 180° C.
The use of amidines as catalysts in PU powder coating compositions is described in EP 0 803 524. Although these catalysts do lead to a reduction in the curing temperature, they exhibit considerable yellowing, which is generally unwanted in the coatings sector. The cause of this yellowing is presumed to be the reactive nitrogen atoms in the amidines, which are able to react with atmospheric oxygen to form N-oxides, which in turn, are responsible for the discoloration. Reference EP 0 803 524 also mentions other catalysts which have been used to date for this purpose, but does not indicate any particular effect of these catalysts on the curing temperature. Such catalysts include the organometallic catalysts known from polyurethane chemistry, such as dibutyltin dilaurate (DBTL), or tertiary amines, such as 1,4-diazabicyclo[2.2.2]octane (DABCO), for example.
Reference WO 00/34355 discloses catalysts based on metal acetylacetonates, such as zinc acetylacetonate, for example. Such catalysts are actually capable of lowering the curing temperature of polyurethane powder coating compositions containing uretdione groups, but their reaction products are principally allophanates (M. Gedan-Smolka, F. Lehmann, D. Lehmann “New Catalysts for the Low Temperature Curing of Uretdione Powder Coatings” International Waterborne, High solids and Powder Coatings Symposium, New Orleans, Feb. 21-23, 2001). Allophanates are the reaction products of one mole of alcohol and two moles of isocyanate, whereas in the conventional urethane chemistry, one mole of alcohol reacts with one mole of isocyanate. As a result of the unwanted formation of allophanate, therefore, isocyanate groups, valuable both technically and economically, are destroyed.
Therefore an object of the present invention is to find high-reactivity polyurethane compositions containing uretdione groups; these compositions being curable even at very low temperatures, and being particularly suitable for producing plastics and high-gloss or matt, light-stable and weather-stable powder coatings and adhesives. Surprisingly, it has been found that certain catalysts so greatly accelerate the unblocking of uretdione groups, that when uretdione-group-containing curing agents are used, it is possible to achieve a considerable reduction in the curing temperature of powder coating compositions or adhesive compositions.
Conventional uretdione-containing coating compositions and adhesive compositions can be cured only at 180° C., or above, under normal conditions (for example, DBTL catalysis). With the aid of the low-temperature-curing coating compositions and adhesive compositions of the invention, it is possible, to have curing temperatures of about 100 to about 160° C., including all temperatures in between, which not only saves on energy and cure time, but also allow for the coating or bonding of many temperature-sensitive substrates, which otherwise if treated at 180° C., or above, would give rise to unwanted yellowing, decomposition and/or embrittlement phenomena. Besides metal, glass, wood, leather, plastics, and MDF board, certain aluminum substrate work very well with the compositions of the invention. Moreover, it is noted that in the case of the aluminum substrates, an excessively high temperature load may sometimes leads to an unwanted change in the crystal structure. A heat-resistant substrate can withstand the temperature(s) at which a coating is applied and cured, without undergoing unwanted changes in shape, color and/or composition.